The present invention relates to a sealable polyolefinic multilayer film, comprising a base layer formed of a propylene polymer and at least one sealable layer which comprises at least one propylene copolymer, at least one low-molecular weight resin and at least one polydiorganosiloxane.
British Pat. No. 1,145,199 discloses heat-sealable laminates which comprise an oriented polypropylene film having at least one heat-sealable layer formed of an ethylene-propylene copolymer comprising from 2 to 6% by weight of ethylene and from 98 to 94% by weight of propylene. These films exhibit good heat-sealibility; however, the films are not scratch-resistant or of the desired degree of clarity, and, in addition, their processing characteristics in high-speed packaging machines are insufficient.
U.S. Pat. No. 4,348,457 describes sealable polypropylene films, which possess a sealing layer comprising an ethylene homo- or copolymer and containing a long-chain aliphatic amine, an incompatible thermoplastic component and a polydialkyl siloxane. Although these films are improved over those of British Pat. No. 1,145,199, their reliability of running in horizontal forming, filling and sealing machines is still insufficient.
U.S. Pat. No. 4,293,608 discloses a packaging material which comprises a base layer of a polypropylene polymer and a surface layer formed of a blend of a propylene-ethylene copolymer and a C.sub.4 to C.sub.10 alpha-olefinpropylene copolymer. This surface layer may also contain a low-molecular weight thermoplastic resin and silicone oils. Packaging materials of this kind have the disadvantage that they are sensitive to scratching and still have insufficient optical properties.
In their brochure "TAFMER XR, Japan 82.03.1000.Cl", Mitsui Petrochemical Industries, Ltd. describe an alpha-olefin copolymer which is suitable for use as a sealing layer for polypropylene films. As shown by Comparative Example 3, polypropylene films provided with sealing layers are, however, still unsatisfactory in many respects.
None of the prior art multilayer films combines all of the characteristics which are important in a packaging film, namely: wide sealing range, low sealing temperature, high gloss, low sensitivity to scratching and good reliability of running in high-speed packaging machines of various types.